Potentiometric electrodes have widespread applications in the fields of biology, chemistry, and medicine. They are used for detecting and measuring the concentration of ion species in solution, the best known example being the pH-meter. The operating principles are well known.
Potentiometric electrodes can also be used as sensors to detect analytes. Analyte may be any desired target that has a corresponding ionophore that is capable of specifically interacting with target substances of interest. Examples of analytes include organic acids, amines, amino alcohols and pharmaceutical drugs. Such electrodes combined with suitable ionophores are capable of detecting a corresponding analyte. The applications of potentiometric electrodes are numerous, including biomedical research, clinical testing, obtaining drug data, industrial pollution testing, food testing and chemical-process control.
At present, there is a strong drive towards application of electrochemical sensing of ionizable substances in miniaturized techniques, such as capillary electrophoresis, and microchip arrays.
Potentiometric electrodes coupled to separation methods (e.g. High Performance Liquid Chromatography, “HPLC”) are practically non-existing. However, such methods can give access to many interesting substances which are available in minute quantities only, and often in admixture with other substances. For these applications, sensors with low specificity are required, in contrast to batch techniques in which high specificity is required.
Patent application EP 0 767 372 A1 discloses a substrate electrode comprising an epoxy resin (or polyester-, or polyimide resin) cylinder containing carbon fibers. A classical poly (vinylchloride) (PVC)-based potentiometric membrane is coated on this substrate electrode. The authors made a sophisticated design to optimize the membrane—substrate electrode contact,
Patent application EP 0 684 466 A2 discloses a substrate electrode which is a solid conductor or semiconductor. It is coated with a classical PVC/plasticiser/ionophore mixture, plus an additional conductive polymer.
Patent application EP 0 300 662 A2 discloses a conventional coated wire potentiometric electrode which is optimized for the measurement of cationic and anionic surfactants.
U.S. Pat. No. 5,552,032 describes a chloride (or generally, a halide ion) sensitive potentiometric electrode. It is a variant of the potentiometric sensors based on silver halide crystal electrode materials.
A problem with potentiometric electrodes of the prior art is the lack of stability (potential drift), mechanical robustness and sensitivity. Their life span is unpredictable, so requiring regular financial expenditure in the form of maintenance on the part of the owner. Furthermore, they are unsuitable for use in HLPC detection and Capillary Electrophoresis (CE) detection, dissolution testing of pharmaceuticals, wherein their behaviour is difficult to predict, and some designs cannot be used with the ion-pairing agent frequently used in HPLC and CE separations or with the detergents used in dissolution testing.
There is clearly a need for a new potentiometric electrode, suitable for use in sensing analytes which overcomes the problems of the prior art.